Phosphorus is an important element, and indeed essential to life. However, the release of phosphate to surface waters, and its consequent contribution to eutrophication, has also led to increasing water quality concerns. Policies were therefore implemented throughout the world, to reduce the levels of phosphorus entering surface waters, by the implementation of technologies to remove phosphorus from domestic and industrial wastewater.
Phosphorus resources are limited and will last about 100 years, if mined by methods currently regarded as economic. This knowledge initiated an interest in technologies which facilitate the recycling and beneficial re-use of the phosphorus present e.g. in waste products in agriculture.
Fertilization with sewage sludge is gradually being prohibited in an increasing number of countries due to the sludge's content of heavy metals and organic contaminants. Incineration is seen as a solution to reduce the volume of disposed sewage sludge.
Ash of incinerated sewage sludge contains about 8-14% P by weight, which is similar to the concentration of P in phosphate rock (e.g. 13% P by weight). The ash commonly contains more than 90% of the P present in sewage. Ash of incinerated MBM (Meat and Bone Meals) contains up to 18% P. Ash of incinerated poultry litter contains about 10% P and phosphorus content in ash of gasified pig manure was reported to be 13% P. The phosphorus present in ash is insoluble in water due to binding with calcium, iron or aluminum. Therefore is the P-fertilizer value of ash low. Furthermore, heavy metals are enriched in ash and limit the recirculation of ash to cropped land. Today, ashes are deposited as a rule.
Phosphorus can be extracted from ashes into an aqueous phase by dissolution with acids or bases.
In summary, several phosphorus containing effluents are formed in various industrial processes, and by the dissolution of ashes and minerals. The effluents are usually dilute and polluted with metals.
There is a need for phosphorus recovery from such effluents. The objective of phosphorus recovery is that it should be used for farming.
Several technologies were developed for extracting phosphorus from domestic and industrial effluents, and from ash leach solutions. The technologies are mainly based on the precipitation of phosphorus as different compounds. However, most such precipitation compounds have a very low solubility and its fertilizer value is low.
However, in e.g. the U.S. Pat. No. 2,850,358, the U.S. Pat. No. 1,879,204, the U.S. Pat. No. 1,835,441, the British patent 410,731 or the translation of the abstract to the Soviet patent 1450266, it is known that tri-ammonium phosphate is more or less insoluble in concentrated aqueous ammonia. An excess of ammonia can then be used to precipitate phosphorus as tri-ammonium phosphate, which can be easily processed to a high quality fertilizer.
However, in order to precipitate phosphorus efficiently with an excess of ammonia, the initial phosphorus concentration must typically be high. Furthermore, a large excess of ammonia is needed. The remaining solution after the precipitation of tri-ammonium phosphate therefore contains large amounts of ammonia which must be treated by e.g. ammonia stripping. Therefore, it is not possible to recover phosphorus from dilute phosphate containing solutions by precipitation of tri-ammonium phosphate in a cost effective way.
In another approach, phosphorus can be separated from the metals by using anion exchange excluding metal cations. The published PCT patent application WO 00/50343 describes a process for recovering phosphorus from ash leach solution using ion exchange.
The approach presented in the disclosure WO 00/50343 has a number of severe drawbacks. The overall efficiency is limited, the process control is complex and the used regeneration solution (hydrochloric acid) gives no added value to the final phosphorus product.
The main limitation of using ion exchange technology as proposed in WO 00/50343 is that the solution recovered during regeneration still has relatively low concentration far below the solubility product. Concentrated regeneration solutions occupy only a small volume of the ion exchange bed and are hence diluted with the solution present in the ion exchange bed. To displace the regeneration solution out of the ion exchange bed requires another solution which thereby dilutes the eluate again. Thus, despite a high initial concentration of the regeneration solution the maximum eluate concentration achieved is often still far too low to be of commercial value.
U.S. Pat. No. 3,579,322 describes the use of Continuous Ion eXchange (CIX) for phosphate recovery from waste effluents formed during the industrial processing of rock phosphate. CIX can achieve a higher eluate concentration than possible with fixed bed ion exchange. However, CIX is a complex process, in which the movement of the resin results in resin abrasion which reduces resin life time. Furthermore, the maximum phosphorus concentration possible with this technology is limited.